(CNN)There is a great, potentially existential, danger lurking deep within the next critical decision President Trump will have to make -- the one he has said would define his presidency: how to do away with ISIS.

It is a decision that petrifies America's allies in Europe, much of the Middle East and far beyond. But it is coming, perhaps by the end of the month. That's the deadline the President has given the Pentagon to come up with its plan. And many of the alternatives are terrifying.

Already, we have seen some of the outlines. A broad range of options: from beefing up existing units that work, largely in the background, training and advising Iraqi forces; adding new, more sophisticated weaponry; all the way up the scale to all-in combat.

This outer limit of that range is, potentially, the most frightening. That's where we get into large numbers of boots on the ground in the uncontrollable hell hole that northern Syria has become. Of course, even the upper end of proposals from the Pentagon are likely to be nothing like the levels reached at the peak of the Iraq war deployments in 2007, when the United States had some 170,000 troops on the ground there.

Still, there are numbers that are on the table, far short of those levels, that could still pose enormous risks now and in the future. And there are many in the Pentagon, even at the highest levels, who are delighted by this opportunity to demonstrate just what the American military can do if it is let fully off its chains.

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The Pentagon's range of recommendations will now have to flow through the President's new National Security Advisor, General H.R. McMaster, appointed by Trump Monday. McMaster is known as a voice of reason within the Pentagon and can hopefully steer the President to an appropriate and measured response to the war on ISIS. His appointment comes in the nick of time.

Last spring, Air Force Gen. Mark A. Welsh III told me, "If you have a stronger military, you have more options. I have no idea what the next President will do."

Two months earlier, then-candidate Donald Trump had already said in a Republican presidential debate, "We really have no choice, we have to knock out ISIS. I would listen to the generals, but I'm hearing numbers of 20,000 to 30,000." Now the generals are about to be heard, and these kinds of numbers are no longer fanciful.

At the lower end of the wide range of options that will likely be presented by the Pentagon, there's a reinforcement of the current level, as CNN Pentagon correspondent Barbara Starr reported, some 5,155 now deployed -- below the allowed ceiling of 5,262, with several hundred more on temporary assignment. But dispatching an entire division, such as the 82nd Airborne, the Army's most mobile fighting unit -- with some 1,700 troops already advising Iraqi forces -- could quickly bring the number deployed in country to 20,000.

Then of course, there's the question of just who we'd be fighting beside? To root out ISIS means a large push into Syria. Will we be fighting with or against the forces of the malignant Syrian dictator Bashar al-Assad? Alongside Russian or Iranian forces? All most thorny -- and again, most potentially deadly -- questions.

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The Pentagon has been thinking about such questions for a long time. As Army Chief of Staff Gen. Mark A. Milley observed in May: "The very first thing that has to be answered is: What's the role of the United States in the world? How do we define ourselves? And I think that question has to be asked and answered before we get into details about tactics and operations."

President Trump, it would appear, has already asked and answered this question. Yet with his White House in considerable disarray, a quick fix of thousands of American paratroopers descending on the ISIS stronghold of Raqqa in northern Syria and wiping its headquarters clean in a single dramatic stroke might appear quite an attractive option. But it may also be precisely the answered prayer for ISIS's wily leaders. ISIS would like nothing more than to have large numbers of American troops within its immediate field of fire. ISIS can then begin sending these US soldiers home in body bags in substantially larger numbers than today, when only scattered "advisors" are deployed among larger numbers of Iraqi troops who bear the brunt of the fighting.

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At the same time, the enhanced presence of American forces would immediately transform the entire nature of the conflict, allowing ISIS propagandists to portray this as a battle between Islam and heathens, rather than the intra-Islamic conflict that has taken shape since the United States exited.

This is what some are calling "the broken glass theory." For the supporters of this view, there are darker worries. The United States has the unquestioned capacity to destroy ISIS in its lairs in Raqqa, this theory goes, leaving behind large piles of broken glass. At the time of the American pullout from Iraq in December 2011, al-Qaeda in Iraq, which appeared to have been all but destroyed, simply went underground, disappeared, and within two years, gopher-like popped up again, in an even more virulent form, in the chaos of northern Syria.

Today, ISIS has many more options for rebirth -- operating directly or in franchised form in 18 countries across the Middle East and Africa. To root it out definitively would require far more than a "surge" of 20,000 troops of the 82nd Airborne. America needs to cultivate allies in the region and let them take up the charge.

The real danger, though, is that the Pentagon will provide President Trump with an off-the-chain option that he will perceive as making good immediately on his campaign pledge to destroy ISIS, with little understanding of the long-term dangers posed by this organization and its attractiveness to new recruits in the Middle East and target countries across Europe and especially the US. Americans will be substantially less safe.

America's allies are also petrified that this is what is waiting in the wings. And the President has done little to reassure them.

One hand giveth, the other taketh away, and no one really knows which hand President Trump may be playing at any given moment. That worked well for Richard Nixon, who enjoyed saying he wanted to be thought of as a little crazy, a little unpredictable, but within boundaries. Today, there do not seem to be any boundaries at all. And this is what the generals, who are in a position to give Trump toys of unspeakably lethal proportions, need to understand.